It would be nice if the quality of screenshots could be tweaked. The jpg-compression is currently too high for my taste, as demonstrated by the comparison below. Maybe three different quality settings, low, medium, high. I would regard the current compression level as "low".
Frame in Virtualdub vs screenshot fetched by PVD (bottom), both frames zoomed 200%.
While I'm at it, there is still room for improvement of the automatic screenshot maker. It fails to create proper screenshots for a lot of movies, not limited to the matroska files. A common error seems to be that all frames are grabbed from the first few seconds of the movie, regardless of setting to "fixed interval" or "random frames". Sometimes they are all grey, and sometimes the scanner just fails to create any (ffmpeg often crashes in those cases). The basic functionality seems to be in place however, I can easily create proper screens for problem files using the manual scanner.
Typical bad screenshot result: